1. Field of the Invention
The present invention relates to a method for manufacturing a three-dimensionally shaped comb-tooth electret electrode.
2. Description of Related Art
An electret that is formed to have mutually opposing planar comb-tooth electrodes is per se known as an electricity generation element that converts vibrational energy into electrical energy. This type of electricity generation element has a construction in which, for example, two comb-tooth structures are provided in two mutually opposing rectangular regions (for example, refer to Japanese Laid-Open Patent Publication 2010-136598) or circular regions (for example, refer to Japanese Laid-Open Patent Publication 2011-072070).
With an electricity generation element employing an electret in which planar comb-tooth electrodes of this type are formed, in order to increase the capacity for electricity generation, it is necessary to make the area great, and accordingly it has been difficult to manufacture a compact electricity generation element whose output is high.
In recent years, by employing the MEMS technique to manufacture the comb-tooth electrodes in a three-dimensional construction, a compact electricity generation element has been developed in which the electrostatic capacity between the opposing electrodes has been increased. For example, mutually opposed three-dimensional comb-tooth electrodes of this type are used in the electricity generation element disclosed in Japanese Laid-Open Patent Publication 2010-011547, and an electret that is provided in the vicinity of the comb-tooth electrodes is used as a means of applying voltage between the electrodes.
Generally, the manufacture of an electret is performed by injecting electric charges into an insulation layer with a corona electrical discharge. However since, with an electret that has been manufactured by this method, there are the problems that the electric charge density in the insulation layer is low, and also that the electric charge level may decrease due to long-term use, accordingly a construction for an electret is desired that can provide a higher electric charge density and also a longer life.
In Japanese Laid-Open Patent Publication 2010-068643, a construction is disclosed for manufacturing an electret having planar comb-tooth electrodes, in which so-called BT treatment (Bias-Temperature treatment) is performed by applying a voltage of around 500 V to 1000 V to an alkali glass while heating it to a temperature that is less than the glass transition point, so that alkali ions within this alkali glass are shifted and a region that is depleted in alkali ions is created.